If federal investigators are hoping to build a public corruption case against former Virginia state senator Phillip P. Puckett or Del. Terry G. Kilgore (R-Scott), they might have an uphill climb, legal experts said.

In 2010, the Supreme Court decided that the government could not prosecute legislators or government officials for what is known as “honest services fraud” simply because they participated in some sort of self-dealing, conflict-of-interest arrangement.

Prosecutors had to prove, the court decided, that the officials took bribes or kickbacks as part of a quid pro quo. Proving that quid pro quo requires prosecutors to show that the public official took or promised to take “official acts” in exchange for the bribes or kickbacks.

Puckett’s resignation last week gave Republicans control of the Virginia Senate and coincided with talks with GOP lawmakers, including Kilgore, involving a job for Puckett at the Virginia Tobacco Indemnification and Community Revitalization Commission and a judgeship for his daughter. In addition to his elected legislative seat, Kilgore is chairman of the tobacco commission.

Two people with knowledge of the investigation confirmed that federal investigators have interviewed officials and sought documentation related to Puckett’s resignation and the jobs.

Puckett, a Democrat from rural Russell County, caused a firestorm when he resigned and prepared to take a job as deputy director of the commission, which Kilgore confirmed at the time had been arranged for the senator. Both Puckett and Kilgore said that the jobs were not offered in exchange for the resignation.

At the time, Kilgore said the resignation simply made Puckett available to take the position, which involves awarding economic development grants funded by the national tobacco litigation settlement.

According to Kilgore, a meeting of the commission’s executive committee was scheduled to take place two days after Puckett’s resignation at which it was expected to consider appointing him. The meeting was canceled after Puckett withdrew from consideration amid the firestorm that his resignation caused.

Kilgore has retained a lawyer, Thomas Cullen, who is a former federal prosecutor in the Western District of Virginia.

The question for Puckett and Kilgore and those investigating them, experts said, is this: Is resigning one’s office an “official act”?

“It’s a stretch, because I think it’s more of an abdication than an act,” said Andrew T. Wise, a defense lawyer at Miller & Chevalier who represented a lobbyist connected to Jack Abramoff. “I would hate to be the line prosecutor trying to sell this to my supervisor.”

But such a prosecution is not impossible, Wise said. Say, for example, a state legislator took $5,000 to abstain from voting on a matter when his or her vote would have broken the tie. Would that constitute honest services fraud, and is that so different from Puckett’s conduct?

“You could cobble together an argument,” Wise said.

Edward T. Kang, a former federal prosecutor now at the Alston & Bird firm, said it is likely that agents are looking for evidence of “official acts.” As to whether the resignation itself could be the official act, Kang said, “That would be novel. I’ve definitely never heard of anything like that.”